"A body continues in its state of rest or of uniform motion unless compelled by an external force to act otherwise."
Newton's First Law of Motion
"When two bodies interact, the forces they exert on each other are equal in magnitude and opposite in direction."
Newton's Third Law of Motion
"A force can be applied at any point on its line of action without altering its external effect on a rigid body."
Principle of Transmissibility
A force may be defined as the push or pull of one body acting on another. It may be concentrated, being considered to act at a point, or distributed, acting over a finite area.
External forces are those caused by the action of other forces and bodies, while internal forces hold a body together. For a car, external forces include friction, weight and the reaction of the Earth, and internal forces include those in the chassis, bolts and seats.
Force (F) is a vector quantity. It therefore has both magnitude and direction. To determine the effect of a force on a body, however, we also need to know its point of application. For external forces, only the magnitude, direction and line of action are needed.
By convention, forces acting in the positive x, y and z directions are positive.
The SI unit of force is the newton (N).